This invention relates to an improved method and apparatus for the utilization of molecular sieves in freeze drying.
Freeze drying is now considered a basic method for the high quality preservation of pharmaceuticals such as vaccines, vitamin preparations, hormones, antibiotics, recombinant DNA products and the like. The food industry finds the method useful for dried convenience foods such as instant coffee, and field rations for the military or for sportsman. Freeze drying involves solidly freezing a high water content material, and then subjecting the frozen material to a high vacuum and controlled heating until substantially all of the original water content is removed. The water is removed via sublimation, i.e. ice goes directly to water vapor, by-passing the intermediary liquid phase. Results are usually a high quality dried product that can be quickly and easily reconstituted to virtually the original product by simply adding water
In standard freeze drying techniques it is necessary to eliminate the sublimating water vapor before it gains entrance to the oil-sealed vacuum pump, since water condensing in the pump oil rapidly causes an unacceptably high pressure rise within the vacuum pump, and hence the entire system. To prevent this, water vapor is frozen out on cold surfaces which are routinely refrigerated by means of mechanical refrigeration compressors, which make use of recirculated refrigerants such as "Freon" (a registered trademark of E. I. duPont deNemours & Co.).
Materials to be freeze dried almost always have to be maintained at temperatures below the freezing point of water during freeze drying, i.e. 0.degree. C., as, for example, -10.degree. C., -20.degree. C. or even lower temperatures. The reason for this is that these materials often contain salts and sugars which give them low "eutectic temperatures", i.e. temperatures at which they are solidly frozen. Above these temperatures these materials might appear be frozen, but small, unfrozen pockets would remain. These pockets would evaporate rather than sublimate during freeze drying, yielding poor to unacceptable results. To enable the frozen material undergoing freeze drying to sublimate at these low temperatures places a second burden on the refrigeration compressors. They must not only efficiently freeze out the sublimating water vapor, but also maintain this ice condensate at a low enough temperature so that the ice within the frozen product (naturally at a warmer temperature than the condensed ice) remains at an acceptably low limit. To do this the mechanical refrigeration compressors must operate at unusually low temperatures of -40.degree. C. or even substantially lower. At these low temperatures mechanical refrigeration compressors tend to be inefficient, and are prone to premature mechanical difficulties.
In a previous application (Ser. No. 738,378, filed May 28, 1985, now U.S. Pat. No. 4,561,191, the disclosure of which is hereby incorporated by reference) I describe a method and apparatus for continuous freeze drying using molecular sieves in place of mechanical refrigeration for sequestering the sublimating water vapor, resulting in increased operational efficiencies and equipment reliablility. However in the preferred embodiment of U.S. Pat/ No. 4,561,191 it is necessary to place the molecular sieves in solid wall metal holders in order to control exothermic heat dissipation during freeze drying, and to assist heat transfer to the sieves during regeneration. It is also necessary to have these metal holders be relatively short and narrow in order to facilitate regeneration of the sieves by heat regeneration without the use of a purge gas. This requires a large number of solid wall metal holders for the molecular sieves, which adds to the complexity and expense of fabricating condensers containing these molecular sieves. Since the molecular sieves are packed solidly in these holders sublimating water vapor flow is impeded, thereby placing relatively low limits on the rate of water vapor flow permissible when low temperatures of the ice within the sample being freeze dried, e.g. -10.degree. C., is required for high quality preservation of the sample. Also, the surface area of molecular sieves within solid wall holders which is quickly available to the sublimating water vapor (for purposes of sequestering said water vapor) is greatly limited. Further, the ability to have the water vapor migrate in all directions, and in particular in the opposite direction from the normal flow of the non-condensable gases (which will be discussed further), is virtually eliminated in the "fully packed" molecular sieve holder configuration.